The present invention relates to a pulper for producing paper pulp from waste paper containing undesired scrap particles. The pulper comprises a container forming a chamber for receiving waste paper and water. The chamber is defined by a bottom wall and a substantially cylindrical circumferential wall. The pulper further comprises a paper tearing member provided in the chamber closed to the bottom wall and rotatable in a horizontal direction of rotation about the centre axis of the cylindrical wall. The bottom wall and the circumferential wall form a substantially circular peripheral reject chute situated under the paper tearing member for collecting reject including undesired scrap particles and extending helically in the direction of rotation of the paper tearing member from an upper end of the reject chute to a lower end of the reject chute, the circumferential wall extending vertically to the reject chute. A discharge pipe is connected to the reject chute at the lower end thereof and a conveyer screw is arranged in the discharge pipe for discharging reject in batches from the reject chute.
Traditionally, pulpers are used in the pulp and paper making industry as a first process step to initially fragmentize and disintegrate the waste paper, which usually is delivered in bales, in water, so that a primary paper pulp is obtained. The primary paper pulp usually contains a great deal of undesired relatively large scrap particles, such as glass, stone, metal, plastic and the like. Such undesired scrap particles are removed from the primary paper pulp already in the pulper with the aid of a strainer that only permits passage of paper pulp and smaller impurities.
A problem of the traditional pulpers however is that also relatively large scrap particles are disintegrated by the rotating paper tearing member into smaller particles that are able to pass through the strainer of the pulper, which makes it more difficult to clean the primary paper pulp in the subsequent cleaning steps. EP 0414602 A2 discloses a known pulper of the kind initially described that partly eliminates this problem. Thus, the container according to the known pulper has a traditional, conically designed wall portion of the circumferential wall extending downwardly to the reject chute, and a reject outlet for continuous discharge of reject from the reject chute. Undesired large scrap particles are intended to separate to the reject chute and be discharged continuously from this via the outlet.
However, the known pulper is not capable of satisfactorily preventing large scrap particles from being disintegrated by the paper tearing member. In addition, some released fibres are lost through the reject outlet, which is a drawback.